To close DEBUG and go back to DOS environment. Tyoe Q and press <ENTER> key.


                                               

Adding and subtracting a-digit hexadecimal Values 

To get the sum and difference of two hexadecimal numbers: type H followed by a space, 
a 4-digit hexadecimal value followed by a space again, another 4-digit hexadecimal value 
and then press <ENTER> key. The leftmost pat is the sum and the rightmost part is the 
difference.
 


Viewing the contents of the registers 

TO show the values of the registers: type R and press <ENTER>key.

                                               

Changing the contents Of a specific register 
TO modify the value if a particular register, let's say BX:type R followed by a space, type

BX and press <ENTER>. Debug Will display the current value of BX (i.e. 0000) and Will

Step 3:Enter these hex codes using the command E 0200. Type E 0200 after the DEBUG prompt and press ENTER key. Key in the first hex code (i.e. 48) and press SPACE bar. Key in the second hex code (i.e. 65) and press SPACE bar. Do it over and over again until you have entered 24 for the dollar sign. Press ENTER key to end the process.

 
The display consist of the three parts.

    The address in segment:offset format. 
    The hexadecimal representation are those that shows (2) hex digit for each byte, followed by a space for readability. Also, a hyphen separates the second 8 bytes from the first 8. 
    The ASCII representation of bytes that contains displayable characters, which can help you, interpret the hex representation. 

Take note that the remaining characters displayed after the dollar (S) sign are the miscellaneous assortment of characters. Whenever you see a period (.) in the ASCII window, it represents either a period or a special character such as the Greek letter alpha. DEBUG's D command displays only 96 out Of 256 characters in the ASCII table so a period is used for the remaining 160 characters. 
Creating a program in DEBUG 
TO construct a program in DEBUG: type A and press <ENTER>key. This command tells DEBUG to begin accepting symbolic assembly instructions and initializes the starting address;
Running the entire program 
To execute the entire symbolic assembly program: type G and press <ENTER> key. DEBUG will display the output and the message "Program terminated normally". Let's have a new set of
program this time, key in the following instructions each followed by pressing <ENTER> key: 
MOV AX, OBEA 
MOV ex, 0103 
ADD AX, BX 
INT 2>0

Saving your program TO save your work in DEBUG follow the procedures below: (Well assume that were going to save the program illustrated above in the same path drive C:.) Step 1:Determine the number of bytes by subtracting the last offset address(before the DEBUG prompt) from 0100. Type H 0108 0100 press CENTER key. Get the rightmost value of the difference (0008) Step 2:Store that value (0008) to CX register. Type R CX press ENTERkey, after the colon type in and pressENTERkey again. Step 3: Give the filename Type N SAMPLE.COM and press ENTER key. Step 4:Save the program onto disk storage by using W commandType W and press ENTER key, the message Writing 0008 bytes Will be displayed On screen.

The machine code to add the values of Ax and BX is Olh and D8h, while to subtract their values is 29h and D8h. These numbers have no meaning to us, but the computer will interpret as add and subtract instructions.

Step 2:  Input the numbers F7h and E3h using E command

Step 3: Display the register and trace if the instruction will work.

In multiplication, when the product greater than 16-bit result, the higher 16-bits are stored in DX register. For division, the quotient is stored AX while the remainder is stored in DX. Now try dividing the sample numbers in your computer.

THE INSTRUCTION SET

The 8086 / 8088 instruction set includes equivalents to the instructions found in the 8086 or almost any 8-bit microprocessor plus some new operations. The instruction categories include:

I.          Data Transfer - that move bytes or words of data between memory and register as well as between accumulator and the I/O parts.

II.         Arithmetic - Capable of adding, subtracting, multiplying and dividing data as either bytes or words.

III.       Program Transfer - Include, Jump, and Call and return instructions familiar in the 8085 as well as some addition instructions that form loops.

IV.       Bit manipulation — to manipulate digits in terms of bit.

V.        String Instruction — used to manipulate string of data in the memory.

VI.       Processor Control — Output of the microprocessor. Instructions enable or disable interrupts modify the flag bits, and synchronized external element.

l. DATA TRANSFER

Data-Addressing Modes

Because the MOV instruction is one of the simplest and most flexible of the 8086/8088 instructions.

The MOV instruction and defines the direction of the data flow.

Notice that the source is far to the right and the destination is next to the OPCODE with the source and destination separated by a comma. ILLUSTRATION:

The instruction MOV AX, BX transfers the contents of the source register (BX) into the destination register (AX). In other words, this instruction copies the contents of the BX register into the AX register is very important to note that the source never changes and the destination almost always changes.

The data-addressing modes used with this instruction include:
Register Addressing - used to transfer the immediate byte or word of data to the destination register.

Immediate Addressing — used to transfer the immediate bytes or word of data to the destination register.

Direct Addressing — transfer a byte or word between the memory and a register. The memory address of the data is stored with the instruction.

Register Indirect Addressing - Transfer a byte or word between a register and the memory location addressed by a register.

Based-plus-Index Addressing — used to transfer a byte or word or between register and the memory location indicated by the sum of a base register and an index register.

II. ARITHMETIC OPERATIONS

a. ADDITION OPERATIONS

b. SUBTRACTION OPERATIONS

a.MULTIPLICATION OPERATIONS
Multiplication, whether 8 or 16 bit, can be either signed (IMIJL) or unsigned (MUL). Multiplication always results in a double-width product. In 8- bit multiplication, whether signed or unsigned, the multiplicand is always in the AL register. Because the programmer can choose only the multiplier, the symbolic coded version of this instruction specifies only one operand. For example MUL BL will multiply AL by BL and leave the 16-bit product in AX.

ILLUSTRATION:

b.DIVISION OPERATIONS

 Like division in the 8086/8088 can also occur on 8-bit or 16-bit numbers. Numbers are always divided into a double-width. For example, an 8- bit division always divided into a 16-bit dividend. As in multiplication, there is no immediate division instruction. A dividend for an 8-bit division is located in the AX register and the divisor is the operand selected for the instruction. The results of an 8-bit division are two 8-bit numbers: the quotient (AL) and the remainder (AH).

ILLUSTRATION:

Ill. PRORAM TRANSFER

               LOOP (Loop Until Complete)

               It controls the execution Of a program segment in a specified number Of times.

               The CX register should contain a count value before starting the loop and

               automatically decrements by One If CX register is not equal to zero it

               transfers to its operand address which points to the Start Of the loop; Otherwise

               it drops through to the next instruction.

               Format:                  LOOP <offset address>

               Example:               LOOP 0108

PROGRAMMING USING DEBUG

Displaying character (uppercase 'A') on screen.

Displaying character (lowercase 'b') twice on screen.

Using control characters (OA — Line feed and OD — Carriage Return) in a screen display.

 Displaying different character to '9') using LOOPS with separator (underscore) horizontally.

Displaying string ('computer') on screen.

Step 1: Code the instruction at —A 0100.

Step 2: Input the hex code of every character of the string at—E 0200.

Step 3: Press ENTER key to go back to the hyphen prompt, then key in —D 0200 and press ENTER key to display the string.